This invention relates to the transmission of events over conductors, preferably for use in digital systems, and in particular to a signaling technique employing three or more conductors.
One well known technique for conveying events within computer systems typically uses a single conductor for the signal. The state of the conductor is raised (or lowered) from a steady state condition, and then returned to the steady state condition. The resulting pulse, or one edge thereof, is used to denote the occurrence of an event. For example, the conductor normally may be at a potential of 0 volts. A potential source then is switchably coupled to the conductor, causing the state of the conductor to change from 0 volts to a higher or lower potential, for example, 3 volts. When the potential source is disconnected, the conductor returns to its 0 volt state. The rising edge of the pulse, the falling edge of the pulse, the interval of higher potential, or even the combination of the rising and falling edges can be used to denote the occurrence of an event.
In another form of prior art signaling, known as differential transmission, a pair of conductors is used which couple the sending and receiving circuits together. In this form of transmission of events, the states of the two lines remain opposite to each other. For example, one line is held at 0 volts while the other line is held at 3 volts. Then to designate the occurrence of an event, the states of the two lines are changed simultaneously. That is, the low line is switched high, while the high line is switched low. These two transitions are used together to designate the occurrence of an event.
These and other forms of signaling suffer from several disadvantages. Some of them are inherently slow because they require the conductor to return to its original state before a subsequent event can occur. In other words there is a time delay to reset the conductor. During this delay no information can be transmitted over the conductor. Another disadvantage is that there is the possibility of ambiguity in the transmission of information. When examining the state of the conductor or conductors at any given instant, one cannot be certain exactly what information is present. For example, using the 3 volt circumstance described above, if the state of the conductor is 1.8 volts, the system within which the conductor functions cannot know whether the conductor is transitioning to a high or to a low condition. The system must wait to determine whether the conductor will be at 0 or at 3 volts, then interpret that information accordingly.